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How about filling your heat storage tank with something like paraffin wax instead of water then having a closed coil of tubing in the wax, as I understand it paraffin wax holds a lot more heat in the phase change of going from a solid to a liquid and the temps that you are looking at seem about right for wax heat storage.
I also think that if there is space that it would be worth having the storage tank in the cabin of the car instead of in the trunk, that way any heat loss is heating and defrosting the inside of the car and the trunk has less insulation then the cabin.
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